1. Field of the Invention
The present invention generally relates to a dry-etching method for making electronic parts such as semiconductor devices or the like, and especially to a dry-etching method for silicon nitride (Si.sub.3 N.sub.4).
2. Description of the Prior Art
In the conventional dry-etching method for silicon materials, especially for Si.sub.3 N.sub.4 film on SiO.sub.2 film or Si.sub.3 N.sub.4 film masked by SiO.sub.2 film, a mixed gas of carbon fluoride such as CF.sub.4 which contains several percent of O.sub.2 has been used. By making a high-frequency electric discharge in such etching gas, F radical reacts to Si and changes it into a volatile material, and etching process proceeds. Namely, an electron produced by the electric discharge reacts to CF.sub.4 and changes it into an F radical, an F ion and an electron. The reaction proceeds by the following reaction as follows: EQU CF.sub.4 +e.fwdarw.CF.sub.3 +F+e (1)
Further, CF.sub.3 reacts with an O radical and number of the F radical increases. The reaction proceeds by the following reaction: ##EQU1## And the F radical reacts with Si, so the etching process proceeds as follows: EQU Si+4F.fwdarw.SiF.sub.4 ( 3)
The conventional dry-etching method, however, has a problem in that it is impossible to selectively etch only Si.sub.3 N.sub.4 film, which is intended to be selectively without hardly etching the SiO.sub.2 base film, the SiO.sub.2 masking film or the photo-resist (hereinafter abridged as P.R) Namely, the combination Si and O in SiO.sub.2 is comparatively stable and hardly separated by the impinging of F radical. However, it is considered that the combination is separated by the impinging O radical, and the separated and retained Si combines with the F radical, and so the etching process proceeds. The reaction proceeds as follows: ##EQU2## Therefore, in the use of mixed gas of carbon fluoride and O, the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 which is obtained is only 2 or 3 (which is defined as ratio of etching rate of Si.sub.3 N.sub.4 /etching rate of SiO.sub.2).
As an alternative method, a case may be considered that the etching gas contains only carbon fluoride in the absence O. However, such case has another problem of C adhering onto the surface of Si.sub.3 N.sub.4 film with the resultant substantial ceasing of the etching process.
On the other hand, another method is considerable, such that a part of F of carbon fluoride is replaced by bromine to selectively etch the Si.sub.3 N.sub.4 film hardly etching the SiO.sub.2 film. It is known that the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 increases by adding several percent of such bromic-replaced gas within the mixed gas of carbon fluoride and O.sub.2 as mentioned above. The Br atom excited by plasma electric discharge is easy to combine with the O atom as follows: EQU Br.sub.2 +O.fwdarw.BrO+Br (5)
In other words, the Br atom has an effect to suppress the O atom which contributes to the etching process. And responding to the suppression of function of the O atom, the number of F radical decreases and the etching rate of Si.sub.3 N.sub.4 also decreases, and furthermore, the etching of SiO.sub.2 is suppressed. As a result, the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 rises.
FIG. 1 shows the characteristic curves of the etching rate of Si.sub.3 N.sub.4 and the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 as a function of the concentration of CF.sub.3 Br (volume percent, namely equal mol %) under the condition of mixing CF.sub.3 Br gas into (CF.sub.4 gas containing 5 Vol % of O.sub.2). In this experiment, the Si.sub.3 N.sub.4 film and SiO.sub.2 film accumulated on the surface of silicon substrate were etched by using the P.R pattern as a mask under the condition that the total quantity of gas flow was 50 SCCM (standard Cubic Centimeter per Minute, which means the quantity of flow in standard state of 273.degree. K. and 760 mmHg.), the pressure of the gas was 0.4 Torr, and electric power of high frequency was 150 W (0.5 w/cm.sup.2). As shown in FIG. 1, responding to the increase of the concentration of CF.sub.3 Br, the etching rate of Si.sub.3 N.sub.4 decreases, and moreover the etching of SiO.sub.2 is suppressed, so the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 increases as shown by chain line curve in FIG. 1. And the characteristic curve of the concentration of CF.sub.3 Br has a peak at the position over twelve percent. In this case, the etching rate of Si.sub.3 N.sub.4 was 960 angstrom per minute and the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 was 6.1 and Si.sub.3 N.sub.4 /P.R was 4.0. As mentioned above, such prior art is known that the mixing of CF.sub.3 Br gas into CF.sub.4 +5% O.sub.2 makes the selection ratio of Si.sub. 3 N.sub.4 /SiO.sub.2 increase to some extent. But there is a strong demand for still higher selection ratio, with increasing the etching rate of Si.sub.3 N.sub.4.